1. Field of the Invention
The present invention relates to a vibration damping device which has an inner cylinder and an outer cylinder arranged such that their axes extend in parallel with each other, with an intermediary of an elastic member connected therebetween.
2. Description of the Related Art
A vibration damper of a type so-called "liquid-containing bush-type vibration damper" is known. It has an outer cylinder and an inner cylinder arranged such that their axes extend in parallel with each other. This type of vibration damper is used mainly in automobiles, in particular as an engine mount for mounting the engine.
This type of vibration damper has an elastic member disposed between the outer and inner cylinders. The elastic member has a pair of liquid chambers which are connected through a passage having a restriction. Vibration of the engine transmitted to one of the liquid chambers causes the liquid in the liquid chamber to be displaced into the other liquid chamber. The displacement of the liquid encounters a resistance as it flows through the restriction so that the vibration is absorbed.
Shake of automotive engine usually takes place at a frequency range around 10 Hz. The above-mentioned known vibration damper exhibits large attenuation of vibration and, hence has superior vibration damping characteristics in a frequency range around 10 Hz. However, this known vibration damping device exhibits a high dynamic spring constant and reduces the vibration damping effect against engine idling vibration which takes place at a range of 20 to 30 Hz. A vibration damping device also is proposed in which a pressure relief means is provided for relieving the pressure in the liquid chamber so as to effectively absorb the idling vibration of the engine. This type of vibration damping device, however, exhibits an inferior attenuation characteristic in the engine shaking frequency range around 10 Hz.